Method to estimate and compensate for nlos bias in time difference of arrival estimate

ABSTRACT

Devices, systems, and method for compensating for non-line-of-sight (“NLOS”) bias in time difference of arrival (“TDOA”) estimates between a first anchor and a second anchor in a network having an obstacle in the line-of-sight therebetween are provided. The systems and methods include transmitting a first packet from a first anchor; indirectly receiving the first packet by a second anchor, then transmitting a second packet by the second anchor; indirectly receiving the second packet by the first anchor; and receiving the first packet and the second packet by a mobile node. The true fly-time of the first or second packets between the first anchor and the second anchor and the bias in time of flight of the first or second packets between the first anchor and the second anchor are estimated. The time difference of arrival at the mobile device between a direct path and an indirect path is further estimated and the NLOS bias in the time difference of arrival estimated at the mobile device is corrected.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/218,070, filed on Sep. 14, 2015, and entitled “Method to Estimateand Compensate for NLOS Bias in Time Difference of Arrival Estimate,”which patent application is hereby incorporated by reference in itsentirety.

FIELD

The present disclosure relates generally to the localization systems andmore particularly to methods and systems to locating objects wirelesslyusing time-of-flight information where there may be no line of sightbetween objects.

BACKGROUND

Time-of-Flight (ToF), also known as Time-of-Arrival (ToA), is often usedto measure the distance between two wireless devices. The distance R canbe easily calculated as fly time t multiplied by the traveling speed ofthe signal, i.e., R=c*t. For a radio frequency signal, c isapproximately 3×10⁸ m/s.

This direct conversion between time and distance is the foundation ofmany ToF based location estimation technologies. If the ToF is knownbetween a device to be localized (DTBL), referred as mobile devicehereinafter, and multiple devices with known locations (referred asanchors, or reference nodes hereinafter), the distances between a mobiledevice and anchors can be computed and subsequently the mobile locationis estimated using multi-lateration, or other techniques. Thelocalization based on ToA has been widely used in many wirelesslocalization systems.

Given that a mobile node, or device, is generally not time-synchronizedto anchors in a given network, a technique called Two-Way TOA (TW-TOA)is commonly used to estimate the location of the mobile device. TW-TOAtechniques may require signals to be transmitted and received by boththe anchor and the mobile device. By doing so, the round trip fly timeis measured and the distance is calculated using the round trip delay asR=c*T/2, where T is the round trip fly-time. Such an implementationusing TW-TOA is bandwidth and energy inefficient because of the largenumber of transmissions needed for each localization operation. A systemusing TW-TOA for localization operations often has a significantcapacity limit, i.e., the total number of nodes, or updates are verylimited.

A more efficient technique for localization than TW-TOA is based onmeasuring the Time-Difference-of-Arrival (TDOA). TDOA estimates thedifferences in the distance from the mobile device to a plurality ofdifferent anchors. The differences in distance are calculated bymeasuring the difference of time when signals arrive at each receiveranchor, which subsequently determines the flight time difference. Thereare a number of methods to realize TDOA-based locationing, such asdownlink TDOA (DL-TDOA), such as the TDOA system described in U.S.Patent Application Publication No. 2015/0156746, ‘Method and System forEstimating the Location of a Receiving Device,’ hereby incorporated byreference, uplink TDOA (UL-TDOA), such as the UL-TDOA system describedin US Patent Application Publication No. 2015/0185309, ‘Method andSystem for Estimating the Location of a Transmitting Device In aWireless Network,’ hereby incorporated by reference, and Beaconsynchronized TDOA (BS-TDOA). The present disclosure describes techniquesto improve the performance of the TDOA systems and can be applicable toall TDOA schemes.

A DL-TDOA system is illustrated in FIGS. 2(a) and 2(b). At the start ofa positioning ranging process, an anchor 2210 can transmit a firstrequest (REQ) packet 2208. One or more anchors 2202 can respond to theREQ packet 2208 by each transmitting response (RSP) packets 2209. A RSPpacket 2209 may only be transmitted by an anchor 2202 after it receivesa REQ packet 2208. A mobile device 2103 can receive both the REQ and theRSP packets 2208, 2209, and can thereby determine time differences ofarrival and thereby estimate its own position.

A UL-TDOA system is illustrated in FIG. 7. The UL-TDOA system caninclude at least one mobile device 3101 and a plurality of anchors 3101.At the start of a localization process, a mobile device, M 3103, cantransmit a REQ packet 3208. One of the anchors in range, for exampleanchor a0 3101 can responds to REQ packet 3208 by transmitting a RSPpacket 3209. Other anchors in range, for example anchors a1-a3 3101, caneach receive both REQ 3208 and RSP 3209 packets to determine the timedifferences of arrival. The location of the mobile device M 3103 canthus be estimated, as described in greater detail in U.S. PatentApplication Publication No. 2015/0185309, ‘Method and System forEstimating the Location of a Transmitting Device In a Wireless Network.’

Another scheme using a hybrid TW-ToA and TDOA, referred to herein asBeacon Synchronized TDOA (BS-TDOA) scheme is illustrated in FIG. 8. TheBS-TDOA system can include at least one mobile device M 4103 and aplurality of anchors 4101, 4104. At the start of the process, one of theanchors, for example a0 4104, can transmit a REQ packet 4208. The mobiledevice, for example M 4103 can, after receiving REQ packet 4208,transmit a response packet RSP 4209. RSP packet 4209 can be received byall anchors 4101. Anchors 4101 can determine the time differencesbetween when each RSP packet 209 is received. The location of the mobiledevice 4103 can then be estimated using the arrival time difference, asdescribed in greater detail in U.S. Pat. No. 8,259,699, ‘Method andsystem for target positioning and tracking in cooperative relaynetworks,’ hereby incorporated by reference in its entirety.

In all three TDOA schemes described above, two types of packets aretransmitted, a request (REQ) packet, and one or more response (RSP)packets. A RSP packet is only transmitted by a device only after itreceives a REQ packet. The difference lies in the devices used totransmit these packets and to measure the time difference of thearrivals.

-   -   In DL-TDOA, an REQ is sent by an anchor, and an RSP is sent by        another anchor. The time differences of arrival are measured by        a mobile.    -   In UL-TDOA, an REQ is sent by a mobile, and one, or more RSP        packets are sent by anchors. The time differences of arrival are        measured by anchors.    -   In BS-TDOA, an REQ is sent by an anchor, and an RSP is sent by a        mobile. The time differences of arrival are measured by anchors.

For current ToA and TDOA localization schemes, the accuracy of thelocation estimate will be affected by the presence of the non-line ofsight (NLOS) measurements. In aforementioned cases, the NLOSmeasurements can introduce a time delay bias between anchor and mobilethat is a factor that needs to be mitigated. Additionally, mobile nodesare often moving within an area covered by a system. As mobile devicesare moving through a system, it is often not possible to avoid theoccurrence of NLOS measurements, though the bias is not alwaysconsistent as the mobile device moves into a more favorable location,the bias will disappear. For TDOA systems, the NLOS bias between anchornodes can additionally have the same effect on accuracy in locationdeterminations. The bias between anchors, however, is persistent asanchors are fixed and thus do not change locations.

In all three cases described above, the time of flight between anchorsis used to estimate the position of mobile devices. Accuratemeasurements of time of flight is necessary is for obtaining accurateposition estimates for the mobile devices.

The bias caused by NLOS packet transmissions can severely degrade theaccuracy of the position estimate of the mobile device. The bias betweenanchors can be especially harmful as the bias is always present for allthe mobile devices. The bias can negatively impact position estimates ofall individual mobile devices using the anchors not within the line ofsight of each other.

To avoid the problems associated with non-line-of-sight bias, it iscommon practice to carefully choose only the anchor pairs that arewithin Line-of-Sight (LOS) to each other as TDOA pairs. This, however,can sometimes be difficult to realize, especially in complicated indoorenvironments. Even in systems where it is possible to carefully choseanchor pairs within LOS, it may require increasing the total number ofanchors needed for the coverage, or may significantly reduce the overallnetwork efficiency. LOS systems may also require time-consuming, manualpairing, which can indirectly increase the installation complexity andcost.

Accordingly, a need exists for systems and methods that allow for areduction in non-line-of-sight signal transmission bias to enhanceposition estimates for mobile devices.

SUMMARY

Systems, devices, and methods are generally provided for performinglocation estimates of mobile nodes. More specifically, the systems,devices, and methods are designed to compensate for systems where thereis limited, or no-line-of-sight between anchor nodes, or between anchornodes and a mobile node. In one exemplary method for compensating fornon-line-of-sight (“NLOS”) bias in time difference of arrival (“TDOA”)estimate between a first anchor and a second anchor in a network havingan obstacle in the line-of-sight therebetween, the method includestransmitting a first packet from a first anchor and indirectly receivingthe first packet by a second anchor. Then the second anchor transmits asecond packet and the first anchors indirectly receives the secondpacket. A mobile node receives both the first packet and the secondpacket. The true fly-time of the first or second packets between thefirst anchor and the second anchor is estimated. Further, the bias intime of flight of the first or second packets between the first anchorand the second anchor is estimated. The time difference of arrival atthe mobile device between a direct path and an indirect path isestimated and the NLOS bias in the time difference of arrival iscorrected at the mobile device.

In some embodiments the estimating and correcting can be performed bythe first and second anchors. In some other embodiments, estimating ofthe fly-time can be performed by a first anchor. Further, in someembodiments, the mobile device can perform the correcting NLOS bias inthe time difference of arrival. In some embodiments, the estimated biascan be filtered. The true locations of the first anchor and the secondanchor can be known. The estimated bias can be embedded in a packet andtransmitted.

In some embodiments the bias between the first and second anchors can becompensated, according to: ΔR_(AB)=t_(AB)*c−R_(AB)=Δt^(A)/2*c−R_(AB).Where t_(AB) can be the time of travel of the first packet from thefirst anchor to the second anchor, and R_(AB) can be the direct pathbetween the first and second anchors. The estimated bias between anchorscan be used directly in estimating a corrected distance difference,according to: ΔR^(C) _(AB)=ΔR^(M) _(AB)−ΔR_(AB). Where ΔR^(M) _(AB) isthe original distance difference measured at the mobile device.

In other embodiments, the true fly-time between the first and secondanchor cam be embedded in a first packet and transmitted. The truelocations of one or both of the first and second anchors can be unknown.The NLOS bias between the first and second anchors can be estimated andupdated a plurality of times. The NLOS bias between the first and secondanchors can be estimated during the initial setup of the network. Insome embodiments the method can further include estimating the positionof the mobile device in an external computing device.

In another exemplary method for compensating for non-line-of-sight(“NLOS”) bias a time difference of arrival estimate, using an uplinktime difference of arrival (“UL-TDOA”) scheme, the method includestransmitting a first packet by a mobile device; receiving the firstpacket by at least one first anchor and then transmitting a secondpacket by the at least one first anchor. The first packet and the secondpacket are received by at least one second anchor in range of the mobiledevice and at least one first anchor. Estimating the time differences ofarrival at the at least one second anchor and correcting the timedifferences of arrival at the at least one second anchor by subtractingthe NLOS bias between the at least one first anchor and at least onesecond anchor.

In some embodiments the method can include estimating the position ofthe mobile device using the corrected time differences of arrival.Estimating the position of the mobile device can be done in an externalcomputing device.

In a further exemplary method to compensate for the non-line-of-sight(“NLOS”) bias, using a beacon synchronized time difference of arrival(“BS-TDOA”) estimate, the method includes transmitting a first packet bya first anchor, receiving the first packet by a mobile node and thentransmitting a second packet by the mobile node. The first packet andthe second packet are received by at least one second anchor that iswithin range of the first anchor and the mobile node. Measuring the timedifferences of arrival between a direct packet transmission path and anindirect packet transmission path by the at least one second node andcorrecting the time differences of arrival with an estimated bias offly-time between the first anchor and at least one second anchor.

In some embodiments the method can include estimating the position ofthe mobile device using the corrected time differences of arrival.Estimating the position of the mobile device can be done in an externalcomputing device.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description is provided with the accompanyingdrawings, in which:

FIG. 1 is a prior art schematic of localization network based on adownlink TDOA scheme;

FIG. 2(a) shows a prior art of a schematic of transmission and receptionof wireless signals;

FIG. 2(b) shows a prior art timing diagram of the transmission andreception of the wireless signals;

FIG. 3(a) illustrates signal transmissions with NLOS path betweenanchors in a DL-TDOA scheme;

FIG. 3(b) illustrates the timing with NLOS path between anchors in aDL-TDOA transmission;

FIGS. 4(a)-(c) are the flow charts illustrating anchors and mobiledevice for a TDOA location operation;

FIG. 5 is a schematic of a bias compensation for network based on uplinkTDOA scheme;

FIG. 6 is a schematic of a bias compensation for network based on beaconsynchronized TDOA scheme;

FIG. 7 is a schematic of a prior art UL-TDOA scheme;

FIG. 8 is a schematic of a prior art BS-TDOA system

DETAILED DESCRIPTION

This present disclosure describes systems and methods to automaticallyestimate the fly-time bias for anchor pairs and compensate for it duringthe position estimate process. As a result, the system can deploy a TDOAnetwork with anchors installed in arbitrary locations and without havingto worry about introducing bias from non-line-of-sight packettransmissions. According to the present disclosure and two anchorswithin range of each other can be used as a TDOA pair. The presentdisclosure can significantly simplify the network planning process as itallows the network to be planned manually and at the same time reducesthe total number of anchor devices, increases the location coverage, andoffers more accurate position services.

FIG. 1 illustrates a prior art Time-of-Flight (TOF) based locationingsystem 1100 which consists of a number of anchor devices 1101 and mobiledevices 1103. In a DL-TDOA configuration, as described in U.S. PatentApplication Publication No. 2015/0156746, which is hereby incorporatedby reference, anchor nodes are grouped in pairs (e.g., anchor pairs1102). Each pair 1102 consists of two anchors 1101 that are within eachother's range. For example, anchors A and B form a pair 1102. Further,anchors A, B, C, and D can additionally be paired into the followingpairs {B,D}, {C,D}, {C,A}, {B,C} and {A,D} if those nodes are withineach other's range. For TDOA location operation, all, or some of theanchor pairs can transmit using RF signals 1112. Alternatively, thepresent disclosure contemplates that additional signal types can be usedwithout departing from the spirit of the present disclosure. The RFsignals 1112 are received by the mobile devices 1103. The reception timeof the signals 1112 are estimated and used to determine the distancedifference from the mobile to anchors respectively. With the distancedifferences, and the anchor locations known, the position of the mobiledevice 1133 can be estimated.

The present application assumes that signals transmitted by radiodevices are in the form of packets. Moreover, the anchors, radio nodes,and mobile devices can be Ultra-Wideband (UWB) radio devices.Alternatively, it is understood that other signal formats can be used aslong as timing information can be extracted. The mobile devices,anchors, and other nodes can be formed together as a single network.

During a downlink TDOA (DL-TDOA) operation, each anchor 2201, 2202 in ananchor pair {A, B} can transmit one packet. FIG. 2(a) illustrates twoanchors 2201, 2202 and one mobile device 2103 in the DL-TDOA operation.The first anchor 2201 can transmit an REQ packet 2208. The second anchor2202 can receive the REQ packet 2208 and then can transmit an RSP packet2209 immediately, or after a brief delay. A mobile node, or mobiledevice, 2103 can receive both the REQ packet 2208 and the RSP packet2209. The mobile node can then measure the time delay between thereception of REQ (t^(M) _(REQ)) and RSP (t^(M) _(RSP)), Δt=t^(M)_(RSP)−t^(M) _(REQ).

The time-of-flight of the signals are illustrated in FIGS. 2(a) and2(b). The REQ packet 2208 travels by distance R_(AM) 2213 to reach amobile device 2103. The REQ packet 2208 additionally travels by distanceR_(AB) 2212 to reach anchor B 2202. Upon receiving REQ packet 2208,anchor B 2202 transmits a packet RSP 2209. The transmitted signal RSP2209 travels by distance R_(BM) 2223 to reach the mobile device 2103.The path from anchor A 2201 to mobile M 2103 is defined as the ‘directpath’, and the path from anchor A 2201 to mobile M 2103 via a secondanchor 2202 (e.g., anchor B) as the ‘indirect path’. The distancedifference between the direct path and indirect path isΔR=(R_(AB)+R_(BM))−R_(AM). The distance difference from the mobiledevice 2103 to different anchors, e.g., ΔR^(M) _(AB)=R_(BM)−R_(AM), canbe used to estimate the position of the mobile device. The distancedifference is estimated as follows:

ΔR ^(M) _(AB) =R _(BM) −R _(AM)=(R _(AB) +R _(BM))−R _(AM) −R _(AB)=Δt*c−R _(AB)  (1)

where Δt (time difference of arrival) is the time elapsed from receptionof REQ 2208 to the reception of RSP 2209 measured by the mobile devicenode M 2103; R_(AB), and Δt can be computed based on the known locationsof anchor A and B. In case the turnaround time at the second anchordevice 2202 is non-zero, it is subtracted from the measured timedifference as well.

FIG. 2(b) shows the timing diagram of the packets. Where t_(AM) is thefly-time of signal from anchor A 2201 to mobile device M 2103 directly(A→M), and where t_(AB), t_(BM) are the fly-times of the signals via theindirect path (A→B→M). Again for simplicity, the turnaround time isassumed to be zero.

Equation (1) can be used to accurately estimate the distance differenceΔR^(M) _(AB). However, as shown in FIG. 3(a) a bias can be present inthe location estimate when the anchors 201, 202 are not in line-of-sight(LOS) from each other. In the case where the LOS path R_(AB) 112 betweenanchor A 201 and anchor B 202 is not available, the signal may reachanchor B 202 via a non-line-of-sight (NLOS) path. As shown in FIG. 3,the NLOS path consists of two segments R_(AC) 311 and R_(BC) 312 betweenanchors and a reflector 333 due to the presence of a physical obstacle300. Obstacle 300 can be, for example, any of a wall, a display, goods,etc. The total travel distance of the signal is greater than the directLOS path, i.e., R_(AC)+R_(BC)>R_(AB). As a result, the overall travelfrom the indirect path has a positive bias ΔR_(AB)=R_(AC)+R_(BC)−R_(AB).This is reflected as a positive bias in the measured time difference ofarrival (TDOA).

In the above discussed NLOS situation, if equation (1) is used directly,this bias ΔR_(AB) will be included in the overall time differencemeasurement Δt, and as a result, the position estimate can besignificantly degraded. Estimating the bias ΔR_(AB) and compensating forit can therefore improve the position estimate accuracy.

FIG. 3(b) shows the timing diagram of the packet transmission andreception at the anchors 201, 202 and the mobile device 103. FIG. 3(b)additionally illustrates how to estimate the bias ΔR_(AB). For example,the sequence of packet transmissions is unchanged, i.e., the firstanchor A 201 transmits a REQ packet 208, the second anchor 202 receivesthe first REQ packet 208. The second anchor then transmits a RSP packet209. The mobile device 103 receives the REQ packet 208, from anchor A201, and the RSP packet 209, from anchor B 202. The mobile device 103can then measure the difference between the direct path, and theindirect path via the second anchor 202.

To estimate the bias, the first anchor 201 measures the time elapsedbetween the transmission of the REQ packet to the reception of the RSPpacket. The first anchor 201 can estimate the round trip fly-time of thesignal via the NLOS path, as Δt^(A)=2*t_(AB). The fly-time from anchor A201 to anchor B 202 is t_(AB)=Δt^(A)/2. The distance bias is thencomputed as

ΔR _(AB) =t _(AB) *c−R _(AB) =Δt ^(A)/2*c−R _(AB)  (2)

Once the estimated bias ΔR_(AB) is calculated, anchor A 201 canbroadcast this information. The estimated bias can be embedded into thefollowing REQ packet sent by the anchor A 201. Assuming that anchors201, 202 are stationary and their locations do not change over time, thebias between an anchor pair does not change. The anchors 201, 202 canimprove the accuracy of the bias estimate by applying filtering to theestimated bias. The bias between anchor pairs {A, B} can be measuredcontinuously, or during the initial network setup. Each anchor 201, 202can store the bias estimates of its neighboring anchors. The estimatedbias, when available, can be included in the REQ or RSP packets 208,209.

The mobile node, after receiving the bias ΔR_(AB), corrects the originalTDOA measurement with the bias, as follows

ΔR ^(C) _(AB) =ΔR ^(M) _(AB) −ΔR _(AB)  (3)

As we can see, ΔR^(C) _(AB)=ΔR^(M) _(AB)−ΔR_(AB)=Δt*c−(R_(AB)+ΔR_(AB)).Equation (3) can be rewritten as

ΔR ^(M) _(AB)=(Δt−Δt ^(A)/2)*c  (4)

Equation (4) expresses the relationship that the distance difference canbe estimated using the measured time difference of arrival at a mobiledevice, and the measured flight time between anchors. It is notnecessary to know the anchor locations to solve for the distancedifference.

FIGS. 4(a)-4(c) illustrate flow diagrams describing the DL-TDOA schemefrom the first anchor (initiating anchor), the second anchor (respondinganchor) and the mobile device. The flow for each of the nodes, e.g.anchor A, anchor B, and the mobile device, can be summarized as follows:

-   -   Initiating anchor (anchor A)        -   transmits REQ packet 402,        -   receives RSP packet 404,        -   estimates the round trip delay Δt^(A) and computes the            distance bias ΔR_(AB) 406,        -   processing the bias estimate 408, and        -   broadcasts the bias (e.g., in the following REQ packet) 410.    -   Responding anchor (anchor B)        -   receives REQ packet 412 and        -   transmits RSP packet 414.    -   Mobile node (mobile device)        -   receives REQ and RSP packets 416,        -   receives anchor bias ΔR or fly-time Δt^(A) 418,        -   measures fly-time difference Δt between the direct path and            indirect path 420,        -   correct the estimated fly-time difference with the anchor            bias ΔR 422, and        -   estimates position using corrected distance differences for            multiple anchor pairs 424.

The above described methods are applicable for other TDOA schemes.Except as described below, or as will be readily appreciated by onehaving ordinary skill in the art, the anchors 201′, 202′, the mobiledevice 103′, and the physical obstacle 300′ are substantially similar tothe anchors 201, 202, mobile device 103, and the physical obstacle 300described above. A detailed description of the structure and functionthereof is thus omitted here for the sake of brevity. For example, inthe case of UL-TDOA, a mobile device 103′ can transmit a REQ packet 208′to all anchors within range of the mobile device 103′. One or more ofthe anchors, e.g. 201′, 202′, upon receiving the REQ packet 208′, cantransmit RSP packets 209′. Anchors 201′, 202′ receive the RSP packets209′. FIG. 5 illustrates the signal path and packet transmission betweenthe two anchors A and B and a mobile device 103′ during the UL-TDOAoperation. As shown, mobile device 103 initially transmits a REQ packet208′; then anchor A 201 receives the REQ packet 208′. Once anchor A 208′receives the REQ packet 208′, anchor A 208′ can then transmit a RSPpacket 209′. Subsequently, anchor B 202 can receive both the REQ and RSPpackets 208, 209. As shown in FIG. 5, a physical obstacle 300′ betweenat least anchors A and B 201, 202, that creates an NLOS bias between theanchors. The distance difference between the mobile device 103′ to thetwo anchors 201′, 202′ is calculated as

ΔR ^(M) _(AB) =R _(BM) −R _(AM) =R _(BM)−(R _(AM) +R _(AB))+R _(AB)=Δt*c−R _(AB)  (5)

where Δt is the measured time elapsed from the reception of REQ packet208′ to the reception of RSP packet 208′ at anchor B 202′.

The NLOS bias has similar impact on the overall estimate of distancedifference as described above. The NLOS bias can be corrected similarly,provided the bias is measured. The anchors 201′, 202′ can perform thebias measurements prior to the UL-TDOA operation, for example during theinitialization of the system. Alternatively, the anchors 201′, 202′ canperform the bias measurements by transmitting an additional packet fromanchor B 202′ back to anchor A 201′ at any time.

If the true fly-time between anchors is known, the NLOS bias can becompensated for. To compensate for the NLOS bias, equation (5) can berewritten as

ΔR ^(C) _(AB) =R _(BM) −R _(AM) =Δt*c+(R _(AB) +ΔR _(AB))  (6)

or

ΔR ^(C) _(AB) =R _(BM) −R _(AM)=(Δt+Δt ^(A)/2)*c  (7)

In a further alternative system, the above described bias compensationcan be applied to BS-TDOA scheme. Except as described below, or as willbe readily appreciated by one having ordinary skill in the art, theanchors 201″, 202′, the mobile device 103″, and the physical obstacle300″ are substantially similar to the anchors 201, 202, mobile device103, and the physical obstacle 300 described above. A detaileddescription of the structure and function thereof is thus omitted herefor the sake of brevity. In a BS-TDOA scheme, as shown in FIG. 6, afirst anchor, or beacon, A 201″ can transmit a REQ packet 208″. Themobile device 103″ can receive the REQ packet 208″ and in response cantransmit a RSP packet 209″. Other anchors within range, e.g. anchor B202″, can receive both the REQ packet 208″ from anchor A 201″, and theRSP packet 209″ from the mobile device 103″. FIG. 6 illustrates oneexample that includes only one additional anchor B 202″. The locationsof both anchors 201″, 202″ is known, therefore anchor B 202″ canestimate the distance difference between the direct path (A→B) and theindirect path (A→M→B) based on the time difference of arrival measuredat anchor B 202″ as Δt,

Δt*c=R _(BM) +R _(AM) −R _(AB)  (8)

or

R _(BM) +R _(AM) =Δt*c+R _(AB)  (9)

If the path between the anchor A 201″ and the anchor B 202″ isnon-line-of-sight, (NLOS), a bias ΔR_(AB) is present in the totaldistance traveled by the signal. This bias is present in the measurementof time of flight.

The correction of the NLOS bias can be applied as

R _(BM) +R _(AM) =Δt*c+(R _(AB) +ΔR _(AB))  (10)

or

R _(BM) +R _(AM) =Δt+Δt ^(A)/2)*c  (11)

Again, the distance, or time, bias between two anchors in systems whereno line of sight exists between nodes can be estimated offline prior tothe BS-TDOA transmissions, or during the BS-TDOA operation by lettingthe second anchor B 202″ transmitting an additional packet. Anchor A201″ can then receive this additional packet, that itself transmitted,and then estimates the round trip delay bias.

The systems and methods described herein can effectively compensate thebias in the time of flight estimation in NLOS channels between anchors.With the calculated bias, the estimated time of flight can besignificantly reduced and subsequently, the position estimate based onthe time-of-flight or time-difference-of-arrival is more accurate. Usingthis technology, anchors in a real time location system can be used inbuildings or locations where non-line-of-sight conditions exist whilemaintaining high accuracy of position estimates of mobile devices basedon TDOA schemes.

Although systems and methods have been described by way of examples ofpreferred embodiments, it is to be understood that various adaptationsand modifications may be made within the spirit of the scope of theconcepts described herein.

1. A method for compensating for non-line-of-sight (“NLOS”) bias in timedifference of arrival (“TDOA”) estimate between a first anchor and asecond anchor in a network having an obstacle in the line-of-sighttherebetween, the method comprising: transmitting a first packet from afirst anchor; indirectly receiving the first packet by a second anchor,then transmitting a second packet by the second anchor; indirectlyreceiving the second packet by the first anchor; receiving the firstpacket and the second packet by a mobile node; estimating the truefly-time of the first or second packets between the first anchor and thesecond anchor; estimating the bias in time of flight of the first orsecond packets between the first anchor and the second anchor;estimating the time difference of arrival at the mobile device between adirect path and an indirect path; correcting the NLOS bias in the timedifference of arrival estimated at the mobile device.
 2. The method ofclaim 1, wherein the estimating and correcting is performed by the firstand second anchors.
 3. The method of claim 1, wherein the estimating offly-time is performed by the first anchor.
 4. The method of claim 1,wherein correcting the NLOS bias in the time difference of arrival isperformed by the mobile device.
 5. The method of claim 1, wherein theestimated bias is filtered.
 6. The method of claim 1, wherein the truelocations of the first anchor and the second anchor are known.
 7. Themethod of claim 1, wherein the estimated bias is embedded in a packetand transmitted.
 8. The method of claim 1, wherein the bias between thefirst and second anchors is compensated, according to:ΔR _(AB) =t _(AB) *c−R _(AB) =Δt ^(A)/2*c−R _(AB), where t_(AB) is thetime of travel of the first packet from the first anchor to the secondanchor, and wherein R_(AB) is the direct path between the first andsecond anchors.
 9. The method of claim 1, wherein the estimated biasbetween anchors is used directly in estimating a corrected distancedifference, according to:ΔR ^(C) _(AB) =ΔR ^(M) _(AB) −ΔR _(AB) wherein ΔR^(M) _(AB) is theoriginal distance difference measured at the mobile device.
 10. Themethod of claim 1, wherein the true fly-time between the first andsecond anchor is embedded in a first packet and transmitted.
 11. Themethod of claim 1, wherein the true locations of one or both of thefirst and second anchors are unknown.
 12. The method of claim 1, whereinthe NLOS bias between the first and second anchors is estimated andupdated a plurality of times.
 13. The method of claim 1, wherein theNLOS bias between the first and second anchors is estimated during theinitial setup of the network.
 14. The method of claim 1 furthercomprising, estimating the position of the mobile device in an externalcomputing device.
 15. A method to compensate for non-line-of-sight(“NLOS”) bias in a time difference of arrival estimate, using an uplinktime difference of arrival (“UL-TDOA”) scheme, the method comprising:transmitting a first packet by a mobile device; receiving the firstpacket by at least one first anchor and then transmitting a secondpacket by the at least one first anchor; receiving the first packet andthe second packet by at least one second anchor in range of the mobiledevice and at least one first anchor; estimating the time differences ofarrival at the at least one second anchor; and correcting the timedifferences of arrival at the at least one second anchor by subtractingthe NLOS bias between the at least one first anchor and at least onesecond anchor.
 16. The method of claim 15 further comprising, estimatingthe position of the mobile device using the corrected time differencesof arrival.
 17. The method of claim 16, wherein estimating the positionof the mobile device is done in an external computing device.
 18. Amethod to compensate for the non-line-of-sight (“NLOS”) bias in a beaconsynchronized time difference of arrival (“BS-TDOA”) estimate, the methodcomprising: transmitting a first packet by a first anchor; receiving thefirst packet by a mobile node and then transmitting a second packet bythe mobile node; receiving the first packet and the second packet by atleast one second anchor that is within range of the first anchor and themobile node; and measuring the time differences of arrival between adirect packet transmission path and an indirect packet transmission pathby the at least one second node; correcting the time differences ofarrival with an estimated bias of fly-time between the first anchor andat least one second anchor.
 19. The method of claim 18 furthercomprising, estimating the position of the mobile device using thecorrected time differences of arrival.
 20. The method of claim 19,wherein estimating the position of the mobile device is done in anexternal computing device.